1. Technical Field
This technology pertains generally to pseudocapacitors, and more particularly to synthetic metal chalcogenides for use in pseudocapacitive applications.
2. Background Discussion
Faster charging batteries are highly desired for portable electronics, electric vehicles, and regenerative energy storage. Traditional Li-ion battery electrodes offer high energy density storage by utilizing reversible redox reactions, but slow ionic diffusion leads to long charging times (about 1 hour) resulting in low power densities and slow rates.
Electrochemical capacitors such as electrochemical double layer capacitors (ELDCs) offer some advantages over batteries, including fast charging times (<1 minute) and long lifetimes (>500,000 cycles). However, ELDCs have low energy densities compared to batteries since they do not involve redox reactions. Pseudocapacitors are a type of electrochemical capacitor that combine the attractive high energy density storage of batteries with the fast rates of ELDCs.
Much of the work on pseudocapacitors has focused on transition oxide based materials. Pseudocapacitive charge storage has been demonstrated in two transition metal sulfides namely TiS2 nanoplatelets, and mesoporous MoS2 thin films. MoS2 is an attractive pseudocapacitive electrode material for its large van der Walls gaps of 6.2 Å in micrometer sized samples, and as large as 6.9 Å in nanostructured samples. The thermodynamically stable phase of MoS2 is the 2H phase in which the molybdenum atoms are coordinated in a trigonal prismatic sulfur environment.
Lithium insertion into the semi-conducting 2H phase of MoS2 induces a phase transition to the metallic 1T phase of MoS2 that is more electronically conductive. Though this 1T-phase has been exploited in hydrogen evolution applications for its superior electronic conductivity to decrease the overpotential required for hydrogen evolution, this desirable property of MoS2 has not been fully utilized in Li-ion battery applications.
MoS2 has been well studied as a high capacity negative electrode material, achieving up to 1290 mAh g−1 when cycled with Li-ions to 0 V vs. Li/Li+. However, when MoS2 is cycled to these very low potentials to achieve these very high capacities, the subsequent electrochemical cycling operates as a lithium-sulfur redox couple that is prone to short lifetimes (<300 cycles). While high capacities are achieved at these low potentials, the parent crystalline atomic structure is completely destroyed and does not reform after the Li is removed. Additionally, MoS2 converts to a highly electronically conducting metallic phase if the voltage is kept higher than 0.8V vs. Li/Li+, so conversion type-reactions cannot utilize this beneficial property of MoS2 (which we exploit in our technology described herein). Furthermore, these diffusive phase changes are kinetically hindered, thereby precluding their use as a fast charging pseudocapacitor, which relies on rapid kinetics.
Given an ideal architecture and morphology consisting of an interconnected mesoporous network, MoS2 thin films exhibit fundamental pseudocapacitive properties. MoS2 can be synthesized with varying degrees of crystallinity which has been shown to strongly influence the electrochemical properties in Nb2O5. MoS2 synthesized by low temperature colloidal synthesis and hydrothermal techniques typically lead to very disordered crystal structures. Another method to synthesize MoS2 nanostructures is thermal conversion of molybdenum oxide precursors in H2S gas. This oxide-to-sulfide conversion is typically performed above 600° C. which can lead to well crystalized MoS2 with good preservation of the preformed nanoscale architecture.
Commercial electrodes for Li-ion batteries are required to have high active material volumetric loadings to maximize the energy density of the cell. In order for this requirement to be satisfied in pseudocapacitive based composite electrodes, close attention also needs to be spent on the electrode architecture. The three main components that comprise a composite electrode are the active material, the conductive additive, and the non-conductive polymeric binder. The optimization of these parameters strongly influences the energy density and power density of the final electrode. A further complication arises from the use of nanoparticle based charge storage materials. Nanoparticles tends to agglomerate making it difficult to disperse within the electrode matrix. This results in electronically resistive inter-particle electrical contact, ultimately leading to poor power density due to inhomogeneous current gradients. Therefore, even if a material itself shows pseudocapacitive characteristics, their characteristic charge/discharge rates cannot be realized if they are embedded in an electrode architecture that is not optimized for pseudocapacitive charge storage.